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 One undoubted drawback is the tendency to deafness inherited by so many. Why the pure white dog should be subject to this defect would need an explanation of too scientific a nature to make it admissible within the scope of this chapter. It may be noted, however, as a matter of interest, that the presence of one or two dark marks on the skin, not apparent in the coat, is sufficient to maintain the hearing unimpaired. The correlation between the total absence of pigment and the non-existence of the sense of hearing has frequently stimulated the investigation of scientists. The same phenonomen is manifested in the feline race, white cats with blue eyes almost invariably being deaf. The curious thing is that if one of the eyes is of a different colour the hearing is not defective. Darwin's omission to speak of the bull terrier when writing upon this topic is not surprising, considering that the all-white dogs were not common in his time. They were a later product.